The present invention relates to an image forming apparatus and an image forming method for reading an image on an original and forming an image using electrophotography.
In a conventional image forming apparatus using electrophotography, the density of an image to be formed is maintained at a constant value by adjusting a charge potential of a photosensitive drum, a development bias potential applied to a developing device, etc., and controlling image formation conditions such as a development contrast potential and a background potential. In particular, in a color image forming apparatus, image formation conditions corresponding to characteristics of respective colors are set, and the density of each color is adjusted to become substantially equal.
In the method of the density adjustment, a toner image (density patch) of non-fixed toner is formed on a photosensitive drum or an intermediate transfer medium through a developing step, and the density of the density patch is measured by a sensor. In addition, environmental conditions within the apparatus are monitored by a system, and the density reproduction state is corrected and an error from a target density value is found. By multiplying the error by a feedback gain, a set-value correction amount within a control loop is computed to find a proper development contrast potential and background potential.
However, in the control for setting a charge potential, a development bias and an exposure amount by computing the proper development contrast potential and background potential, complex calculations are required in finding these values. In addition, in order to confirm whether an error from a target value is corrected for the density patch after density adjustment, it is necessary to form a density patch once again and measure the density patch. Then, if the error from the target value is not yet completely corrected, an error between the density patch and the target value is found and multiplied by a feedback gain. Thereby, a set-value correction amount is computed and a development contrast potential and a background potential within the control loop are found once again.
This process requires a relatively long time, although it depends on the method of forming the density patch. If environmental conditions within the apparatus vary within such a time period, the density reproduction state of the density patch would also vary and an error from the target value may not be corrected. This may lead to an excessively long processing time. A reason for this is that moisture absorption of a developer, which varies due to humidity in the apparatus, varies the density on the density patch, and this phenomenon is conspicuous. In particular, in a color image forming apparatus, density variations of density patches due to moisture absorption of respective color developers are non-uniform. As a result, a difference in density becomes conspicuous.